1. Field of the Invention
The invention relates to the purification of aluminum by segregation. In particular it relates to processes and devices for purification by segregation capable of providing very high purity aluminum, namely aluminum with a purity exceeding 3N 5, in other words containing more than 99.95% of aluminum.
2. Discussion of the Background
Aluminum purification processes by segregation are designed to obtain aluminum with a low content of eutectic elements such as copper, iron, silicon, magnesium, gallium or zinc. These processes are based on the fact that partial solidification of a mass of impure liquid aluminum (called the parent or mother liquor) tends to concentrate eutectic elements in the liquid mass and to produce crystals with a content of eutectic elements significantly lower than that of the parent liquor. Thus, the basic principle of known segregation processes consists of encouraging partial crystallization of a mass of impure liquid aluminum, and physically separating the two phases so as to isolate the purified metal from the residual parent liquor enriched in eutectic elements.
Several processes have been developed around this basic principle. Typically, after placing the charge of impure liquid aluminum in a thermally insulated refractory receptacle, the formation of fine crystals is induced in the upper part of the liquid aluminum by local cooling of the liquid aluminum, and a gradual accumulation of crystals is then induced in the lower part of the receptacle under the effect of gravity, and the purification process is stopped when a given fraction of the parent liquor has been crystallized and the purified crystals are then separated from the residual parent liquor.
According to American patent U.S. Pat. No. 3,211,547, the crystals are formed on the surface of the mass of liquid aluminum by controlled cooling of the mass, and then detach and accumulate at the bottom of the receptacle under the action of gravity. Partial tamping (or compaction) of the mass of crystals takes place at regular intervals throughout the crystallization phase. Compaction of the mass of crystals accumulated at the bottom of the receptacle during crystallization can significantly improve the purification rate achieved by the process. At the end of the crystallization phase, all residual parent liquor is evacuated by pouring in the liquid phase, preferably through an orifice located at the surface above the mass of purified aluminum crystals accumulated at the bottom of the receptacle, and the said accumulated mass of crystals is then recovered by melting it and allowing it to flow in the liquid phase through an orifice located near the bottom of the receptacle.
According to French patent FR 1 594 154 (corresponding to American patent U.S. Pat. No. 3,671,229), crystals form on the external surface of a closed graphite tube adjacent to the part immersed in the parent liquor. A cooling gas circulates in the said tube to trigger crystallization. A graphite ring held in the liquid aluminum periodically scrapes the outside surface of the tube, thus detaching crystals formed on it. The crystals accumulate at the bottom of the crucible under the effect of gravity and the accumulated mass of crystals is regularly compacted using the said ring. The tube is gradually raised as solidification progresses. At the end of the crystallization phase, the purified solid mass (called the xe2x80x9cbloomxe2x80x9d) is then separated from the residual liquid mass enriched in eutectic elements, for example by siphoning of the residual parent liquor or by tipping the crucible. According to French patent FR 2 592 663 (corresponding to American patent U.S. Pat. No. 4,744,823), the purification coefficient of this process can be further increased by tipping the receptacle to allow the residual parent liquor to pour off, and keeping it in the tipped positioned to eliminate the residual interstitial liquid by dripping.
According to French patent FR 2 524 489 (corresponding to American patent U.S. Pat. No. 4,456,480) and American patent U.S. Pat. No. 4,221,590, the crystals accumulated at the bottom of the crucible during the crystallization and crystal compaction phase are remelted, which induces additional purification of the metal which can give purification coefficients exceeding theoretical values.
According to Japanese patent JP 58-167733, the purified aluminum crystals are formed at the periphery of the internal surface of the crucible in a determined area located below the free surface of the liquid aluminum, by means of a cooling device comprising a stainless steel pipe in which cool air circulates. The part of the liquid aluminum located above the cooling area is heated to prevent it from solidifying. Crystals formed in the forced cooling area are detached using a graphite piston with a cross-section approximately the same as the cross-section of the crucible and continuously immersed in the liquid aluminum, which periodically scrapes the surface of the crucible and encourages their accumulation at the bottom of the crucible. The piston is also used to compact the crystals accumulated at the bottom of the crucible. Ducts formed in the piston enable crystals to flow towards the bottom of the crucible and liquid metal to flow during piston movements. The mass of solidified metal is partially remelted using heating means. When the mass of accumulated crystals has reached the cooling area, the piston is emersed, the parent liquor is withdrawn by siphoning and the solid mass is extracted from the crucible, and then cut as a function of the required purity.
The purity of blooms produced industrially according to the state of the art is non-uniform. In particular, a purity gradient is observed between the top and the bottom of the blooms. It is known that the top part of the final bloom that contains more impurities than the lower part can be sawn, to keep only the lower part for applications requiring the highest purity levels. Typically, the sawing operation eliminates 15% to 30% of the final bloom. However, this solution has the disadvantage that it eliminates a large proportion of the blooms obtained, consequently reducing the effective productivity of a plant and generating scrap that complicates metal stock management.
The present inventor has also observed that blooms obtained industrially usually have purity variations between the core and the periphery, the metal at the periphery being purer than the metal in the core of the bloom. In general, the higher activity of large crucibles varies inversely with the purity of the product obtained. For example, the effective purification coefficients K (over the entire bloom) observed for iron (KFe) and for silicon (KSi) on 800 mm diameter crucibles, were often less than 50% of the purification coefficients obtained (at an identical rise speed) for 600 mm diameter crucibles. It can be difficult to extract the high purity metal by a simple sawing operation with this type of essentially radial heterogeneity.
Apart from these heterogeneities, the present inventor has observed significant variations in the time necessary to obtain a bloom with predetermined mass, between one operation and the next. These variations, which are due to different causes (such as equipment wear and variations in thermal conductivity) affect the effective productivity of an industrial site and complicate the work organization and internal procedures.
Finally, the present inventor has observed that the productivity and the average effective purification rate vary inversely to each other. Thus, a reduction in the purification rate is observed when the productivity is increased, and conversely the productivity drops when the purification rate is increased. This constraint significantly limits the margin of maneuver in industrial production and also adversely affects production costs.
Therefore the present inventor searched for solutions in simple devices and processes that could widen the limits of the compromise between the purification rate and productivity, and reduce heterogeneities and purity variations in industrially obtained blooms, in order to globally reduce investment, production and maintenance costs.
The present invention provides a process for the purification of aluminum by segregation in order to form a solid mass (or bloom) with very high purity (namely greater than 3N5, in other words containing more than 99.95% of aluminum) starting from a mass of impure liquid aluminum called the xe2x80x9cparent liquorxe2x80x9d, comprising the formation of crystals by partial crystallization, periodic compaction of the bloom and essentially continuous remelting of the bloom by heating during growth, and characterized in that it comprises a periodic measurement of the height H of the bloom during growth and adjustment of the heating power as a function of the measured height H.
The present invention also provides a device for the purification of aluminum, capable of forming a mass of very high purity solid aluminum (or xe2x80x9cbloomxe2x80x9d) by segregation, comprising a refractory crucible, a furnace equipped with crucible heating means, means of remelting the said bloom by heating during growth, and at least one compaction means, and characterized in that it comprises means of measuring the height H of the said solid mass during growth, and means for controlling the heating power of the said heating means as a function of the said measured height H.
The present inventor has observed that, unexpectedly, heterogeneities in the purity of the final bloom and the variability from one bloom to another were actually related to the heating power injected during the segregation process and that prior practice that consisted of injecting an approximately constant heating power without retroactive (feedback) adjustment during growth of the bloom, led to significantly greater heterogeneities and purity variations than are observed when the heating power is varied gradually according to the invention. The present inventor also observed that control of the heating power throughout the bloom growth period not only reduced heterogeneities and variations in the purity, but also widened the limits of the compromise between the purification rate and the productivity.